Head-mounted displays (HMD) have found myriads of applications from scientific visualization to engineering applications, from medical to defense industries, and from information display to entertainment. A key component to an HMD system is an eyepiece optics that collects the light emitted by a microdisplay and forms a magnified, distant image of the digital information presented through the microdisplay for the eye to view. On the other hand, designing a wide field of view (FOV), compact, low F-number and nonintrusive HMD with a large exit pupil has been a great challenge. The typical eyepiece structure using rotationally symmetric components has limitations in achieving low F-number, large eye relief, and wide FOV.
Many methods have been explored to achieve an HMD optical system which fulfils the above mentioned requirements. These methods include applying catadioptric techniques, introducing new elements such as aspherical surfaces, holographic and diffractive optical components, exploring new design principles such as using projection optics to replace an eyepiece or microscope type lens system in a conventional HMD design, and introducing tilt and decenter or even freeform surfaces. (Morishima et al., “The design of off-axial optical system consisting of aspherical mirrors without rotational symmetry,” 20th Optical Symposium, Extended Abstracts, 21, pp.53-56, 1995. H. Hoshi, et .al, “Off-axial HMD optical system consisting of aspherical surfaces without rotational symmetry,” SPIE Vol. 2653, 234, 1996. S. Yamazaki, et al., “Thin wide-field-of-view HMD with freeform-surface prism and applications,” Proc. SPIE, Vol. 3639, 453, 1999. Dewen Cheng et al, “Design of an optical see-through head-mounted display with a low f-number and large field of view using a freeform prism,” Applied Optics, 2009. Dewen Cheng, et al. “Design of a wide-angle, lightweight head-mounted display using freeform optics tiling,” Optics Letters, 2011. H. Hua, X. Hu, and C. Gao, “A high-resolution optical see-through head-mounted display with eyetracking capability,” Optics Express, 2013.) Among the different methods mentioned above, freeform optical technology has demonstrated great promise in designing compact HMD systems. In particular, a wedge-shaped freeform prism takes advantage of total internal reflection (TIR), which helps minimize light loss and improve the brightness and contrast of the displayed images.
The concept of freeform HMD designs with a wedge-shaped prism was first presented by Morishima et al. in 1995, and the fabrication and evaluation method were explored by Inoguchi et al. (“Fabrication and evaluation of HMD optical system consisting of aspherical mirrors without rotation symmetry,” Japan Optics '95, Extended Abstracts, 20pB06, pp. 19-20, 1995). Following these pioneering efforts, many attempts have been made to design HMDs using freeform surfaces, particularly designs based on a wedge-shaped prism (U.S. Pat. Nos. 5,699,194, 5,701,202, 5,706,136). For instance, Hoshi. et al. presented a freeform prism design offering an FOV of 34° and a thickness of 15 mm; Yamazaki et al. described a 51° optical see-through HMD (OST-HMD) design consisting of a freeform prism and an auxiliary lens attached to the prism; more recently Cheng et al demonstrated a 53° OST-HMD design with low F-number [U.S. Pat. No. 9,239,453 B2], and Hua et al presented the design of a high-resolution OST-HMD design integrated with eyetracking capability [Hua, Hu, and Gao, Optics Express, 21(25): 30993-30998, December 2013].
FIG. 1 shows a schematic layout of a typical freeform prism eyepiece design consisting of three optical surfaces, labeled as S1, S2, and S3. The prism eyepiece serves as the near-to-eye viewing optics that magnifies the image displayed through a microdisplay. For the sake of convenience, the surface adjacent to the exit pupil was labeled as S1 in the refraction path and as S1′ in the reflection path. The center of the exit pupil was set as the origin of the global coordinate system and the rest of the surfaces were specified with respect to this global reference. The inventors further adopted the convention of tracing the system backward, namely from the eye position to the microdisplay. The overall system was set to be symmetric about the YOZ plane, but not the XOZ plane. In FIG. 1 the Z-axis is along the viewing direction, X-axis is parallel to the horizontal direction aligning with interpupilary direction, and the Y-axis is in the vertical direction aligning with the head orientation. A ray emitted from a point on the microdisplay is first refracted by the surface S3 next to the microdisplay. After two consecutive reflections by the surfaces S1′ and S2, the ray is transmitted through the surface S1 and reaches the exit pupil of the system. To enable optical see-through capability, an auxiliary lens may be cemented to the wedge-shaped prism-lens in order to minimize the ray shift and distortion introduced to the rays from a real-world scene when the auxiliary freeform lens is combined with the prism-lens.
Most of the existing wedge-prism-based eyepiece designs have several limitations. First of all, the exit pupil diameter (EPD) of most existing designs is typically from 4 to 8 mm, which essentially results in a limited eyebox size. The eyebox defines a 3D volume in which the pupil of a viewer is placed to see the entire field of view of the display without losing imagery. A larger eyebox is preferred for HMD systems to facilitate ease of use and comfort. Secondly, in most of the existing designs, the size of the microdisplays is relatively large, in the range of 0.8 to 1.3 inches, which affords a relatively large focal length or low optical power to achieve a typical 40-degree FOV. Even with an exit pupil of 8 mm, the system F/# remains fairly high (greater than 3) and eases the optical design challenge. A large size microdisplay, however, offsets the advantage the compactness of using a freeform prism. In the more recent design by Cheng et al (AO 2009), smaller microdisplays, typically around 0.6″, were adopted to achieve a 53-degree FOV, which requires a focal length of ˜15 mm. The substantially reduced focal length makes it very challenging to design a system with a large exit pupil and long eye clearance distance. As a result, the conventional design compromises the size of non-vignetted exit pupil diameter (about 6 mm) by allowing a significant amount of vignetting for large field positions, which compromises the effective eyebox size to about 8 mm at the designed eye clearance position.
Thirdly, the pixel size of the microdisplays used in most of the existing designs is typically at least 15 μm or larger. As a result, relatively low optical power or long focal length is adequate for the eyepiece prism to achieve a moderate FOV. For instance, the pixel size of the microdisplay used in the design by Cheng et al is about 15 μm, which helps to mitigate the challenge of designing a large FOV system. In the more recent designs by Hua et al (2013), microdisplays with pixel size of around 10 μm, were adopted, which requires the freeform eyepiece to afford much higher optical resolution (e.g. 50 lps/mm for 10 μm pixels) than designs with larger pixel sizes (e.g. 33 lps/mm for 15 μm pixels). On the other hand, the microdisplays used in the design by Hua et al are about 0.8″, which helps to mitigate the challenges of designing a high resolution system. In general, it is very challenging to design a freeform prism eyepiece achieving low F-number and high optical resolution for a broad spectrum without adding additional elements for chromatic aberration correction.
Finally, the freeform prism typically is symmetric about the plane in which the surfaces are rotated and decentered and the optical path is folded. For instance, the prism schematic in FIG. 1 was set to be symmetric about the vertical YOZ plane. The optical surfaces are decentered along the vertical Y-axis and rotated about the horizontal X-axis so that the optical path is folded in the vertical YOZ plane to form a prism structure. With this type of plane-symmetry structure, it is very challenging to achieve a wider field of view for the folding direction than the direction with symmetry. Therefore, most of the existing freeform prism eyepiece designs, including the recent work by Cheng et al [AO 2009] and Hua et al [Optics Express 2013], choose to fold the optical path in the direction corresponding to the direction of narrower FOV as shown in FIG. 1, which makes it easier to achieve total internal reflection (TIR) in surface S1′ and maintain a valid prism structure. As most display applications typically prefer a landscape-type display, HMD systems typically align the wider FOV direction horizontally and the narrower FOV direction vertically. As a result, most of the freeform prism-based HMDs typically mount the microdisplays above the eyebrow, which leads to a front-heavy system and compromises the ergonomic design. Prism designs that fold the optical path along the wider FOV direction will allow mounting the microdisplays on the temple sides and mitigating the ergonomic challenge. In the prior art, there are a few exceptions where the freeform prism designs were folded in the direction corresponding to the wider FOV. For instance, Hu and Hua presented the design of a high-resolution freeform wedge prism which was folded in the wider FOV direction so that the prism was mounted horizontally [Hu and Hua, “High-resolution optical see-through multi-focal plane head-mounted display using freeform wedge prism,” Optics Express, May 2014. Hu and Hua, “Design and tolerance of a freeform optical system for an optical see-through multi-focal plane display,” Applied Optics, 2015.]. However, the microdisplay utilized in the system has larger pixels (about 15 μm) and larger dimensions (about 0.7″ diagonally) and the system has a relatively smaller exit pupil (about 6 mm) than the present invention.
The existing body of work shows that it remains a great challenge to design a freeform eyepiece prism offering a wide field of view, high image resolution, large exit pupil for eye placement, sufficient eye clearance, and elegant ergonomic design. Accordingly, it would be an advance in the field of head-mounted displays and near-to-eye systems to provide an eyepiece design which overcomes these limitations.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.